The PRD began as a rural region with an agriculture-based economy. Urbanization didn't take off until the early 1990s, when factories and big business flocked to the region and brought with them millions of new residents.

The changes were already visible over a decade ago. This NASA Landsat satellite view of the PRD shows what it looked like in 1973, when it was still rural, and in 2003, when it had already begun the rapid transition into an urban center.

In 2008, the Chinese government revealed its plan to combine the nine major cities in the PRD — Shenzhen, Dongguan, Huizhou, Zhuhai, Zhongshan, Jiangmen, Guangzhou, Foshan, and Zhaoqing.

By 2030, China plans to spend RMB 2 trillion ($322 billion) in the hopes of drawing more people from the outskirts of the megalopolis to inside its border. The population is expected to rise to 80 million, with a total gross domestic product of $2 trillion. (The entire US GDP is just shy of $17 trillion.)

A man walking at low tide at Lau Fau Shan on Friday with the fast-developing city of Shenzhen on mainland China seen in the background. Bobby Yip/Reuters

The key question is whether China can make the megacity merger sustainable. The country has started construction on bridges and railways to connect the cities, with one another and the neighboring metropolises.

In a sense, the region has a lot of room for growth.

The Hong Kong-Zhuhai-Macao Bridge (HZMB), which will link the three cities in the Pearl River Delta, seen under construction off Hong Kong's Lantau Island, China, on June 12. Bobby Yip/Reuters

Roughly 64% of people in the PRD still live in nonurban areas, according to data from a recent World Bank report.

But the consequences of urbanization — lagging public-transportation infrastructure and a scarcity of affordable public housing — will no doubt leave others stranded on the fringes. And there are also those who will be displaced by rapid construction.

This woman, for example, was swept up in the consequences of large-scale urbanization when workers accidentally knocked her house down during demolition.

Huang Sufangshe seeing part of her house being taken down by demolition workers at Yangji village in central Guangzhou city, Guangdong province. Stringer China/Reuters

In other parts of the PRD, urbanization proceeds at unequal rates, leading to odd juxtapositions in housing complexes, such as this small structure surrounded by high-rises.

Migrant workers chatting at their temporary house near a construction site in Guangzhou, Guangdong province. Joe Tan/Reuters

It will be a challenge in the remaining 15 years of China's expansion plan to resolve pockets of inequality. East Asia already has eight megacities and 123 cities of between 1 million and 10 million people.

If trends hold, those numbers will only continue to rise.

"While this transformation is going on, there is still an opportunity to set the course of urbanization on a more sustainable and equitable path," the World Bank report states. "Within a few decades, this window of opportunity will close, and future generations will be left to deal with the consequences of how we urbanize today."